1. Field of the Invention
This invention relates to loudspeakers and, more particularly, to flat loudspeakers having a planar rectangular configuration.
Most speakers are configured with a cone shaped diaphragm attached to an electromagnetic driver assembly. However, conventional expedients have required massive speaker enclosures in order to increase the efficiency of the speaker, or to increase the quality and bandwidth of sound emitted by such speakers. Many alternative speaker designs have been proposed to reduce the size, and particularly the thickness, of speakers. Although the use of such expedients may permit a reduction in the thickness of a given speaker, they generally do not produce the same quality or output level of sound as do conventional cone speakers.
Recently there has become a need to produce inexpensive, thin compact speakers that are extremely resistant to harsh environmental conditions, and capable of producing a high output sound level over a wide bandwidth throughout the life span of the speaker. Such applications include the automotive industry, computer industry, and the like. The previous expedients in speaker development have generally been unable to meet this need.
Thus, there has been recognized a need to make more compact planar or flat type speakers for use in restricted areas, which also have the ability to produce sounds at a high output sound level over a wide bandwidth throughout the life span of the speaker.
2. Description of the Prior Art
The most common speaker driver assembly for conventional speaker utilizes a voice coil and permanent magnet attached to a cone diaphragm wherein the passage of a fluctuating electrical current through the voice coil causes the diaphragm to vibrate. As the diaphragm vibrates, airwaves are produced which are perceived as sound. Conventional voice coil drive units with conventional cone speakers are highly inefficient, converting less than about five percent of the applied electrical energy into sound energy. Attempts to improve the efficiency of these units have undesirably required massive speaker enclosures, and the like.
Large speakers have many disadvantages. For instance, the large mechanical inertia inherent in these speakers reduces the frequency range at which they can vibrate, which in turn reduces the bandwidth of sounds they can produce. Another disadvantage is that these speakers cannot be used in applications requiring installation in highly restricted and compact areas. Such applications, for example, in automobile door panels, and the like, typically require relatively flat and compact speaker configurations.
An alternative speaker driver to the voice coil assembly is the piezoelectric transducer. The piezoelectric transducer utilizes crystalline materials that mechanically vibrate when subjected to a supplied voltage. Although the piezoelectric type speaker has the ability to be used in more compact speaker configurations, the crystalline vibrations produced generally are unable to produce a practical level of sound output and wide bandwidth of reproducible sound. Hence, piezoelectric transducer speakers, by themselves, have generally been unable to achieve the high level of sound output and quality of sound reproduction required in many space-restricted applications.
Another alternative speaker drive assembly is the electrostatic driver, which uses a sheet or film as a sound radiator coupled with a flat plate or mesh. Generally, the film and plate act together as a capacitor. An audio signal is mixed with a high DC polarized voltage that is applied across the capacitor. When the high DC polarized voltage is varied in accordance with the audio signal; the electrostatic charge across the capacitor varies. As the charge varies, so too does the force between the plates, which in turn causes the film to vibrate. However, the electrostatic driver requires an expensive DC voltage source and transformer to operate, which, in turn, increases the production cost and size of the speaker. Hence, electrostatic speakers are inherently both costly and bulky and are generally unacceptable not only for general applications, but even more so in space-restricted applications.
One relatively compact flat speaker expedient utilizes a solid panel as a sound resonator driven by a direct connection to either a conventional voice coil or piezoelectric driver. However, it is difficult for the solid panel resonator to produce a wide sound bandwidth unless its vibration characteristics conform to a complex bending behavior. In order to configure the rigid panel to respond accordingly, the panel must be precisely manufactured and assembled to exacting tolerances. This is not only time consuming but costly, and is highly undesirable in speaker design. Thus, the use of the rigid panel flat speaker is unacceptable in space-restricted applications.
Another prior flat speaker design utilizes a single thin sheet or film membrane that is pre-stressed in tension within a frame. The single thin sheet functions as a sound resonator. Although the thin membrane eliminates the expense of the rigid panel diaphragm, it too has its drawbacks. For instance, it is difficult to obtain the proper pre-stress during assembly. In addition, the pre-stress must remain essentially constant throughout the life span of the speaker in order to produce quality audio performance over time. Maintaining this pre-stress is difficult, as aging and thermal effects on the film membrane tend to substantially reduce the amount of pre-stress over time. Another drawback with the thin membrane speaker is that it is highly vulnerable to physical damage such as punctures that can significantly reduce the sound quality of the speaker. Thus, the thin film membrane flat speaker, although useable in space restricted applications, does not satisfactorily produce high quality sound output consistently and repeatably over the life span of the speaker.
Previously proposed expedients include, for example, Yokoyama U.S. Pat. No. 5,009,281. Yokoyama proposes several embodiments of acoustic apparatus where the diaphragm of a vibrator radiates directly and also drives a resonator. The disclosed resonators are in the form of chambers, not flat panels. Yokoyama also includes a catalog like listing of prior art transducers. Polk U.S. Pat. No. 4,903,300 discloses a flat speaker for use within wall cavities, but uses the entire volume of the wall space to get the desired output. Kumada et al. U.S. Pat. No. 4,352,961 discloses a flat speaker where a piezoelectric driver is used in a watch. The driver is mounted to the transparent face of the watch, which is used as the resonator. Another thin profile audio device with a piezoelectric driver is shown in Kumada U.S. Pat. No. 4,471,258. Skaggs U.S. Pat. No. 4,714,133 discloses a speaker structure where a conventional cone speaker is acoustically coupled to a radiator. Kasai et al. U.S. Pat. No. 4,551,849 discloses a thin automotive audio system uses a vehicle panel that is directly driven by a driver. Yanagishima et al. U.S. Pat. No. 4,514,599 likewise discloses an automotive vehicle audio system in which a vehicle panel is driven by a driver of the speaker. Watters et al. U.S. Pat. No. 3,347,335 proposes the use of a honeycomb core sandwiched between two stiff sheets as a flat acoustic radiator. Matsuda et al. U.S. Pat. No. 4,122,314 discloses a loudspeaker with a plane vibrating diaphragm where the diaphragm is in the form of a sandwich structure. Guenther et al. U.S. Pat. No. 6,097,829 discloses a flat-Plane diaphragm fabricated using sandwich construction. Barlow U.S. Pat. No. 3,111,187 likewise discloses a flat panel diaphragm fabricated using sandwich construction. Pearson U.S. Pat. No. 3,861,495 discloses a loudspeaker in which a cone speaker is acoustically coupled through telescoping frusto-conical members to a flat vibrating panel. Murase U.S. Pat. No. 3,674,109 discloses a thermoplastic laminated vibration plate for a loudspeaker, which includes a centrally located cone portion and a flat portion surrounding the cone portion. The cone portion is only a fraction of the whole diaphragm area. Matsuda et al. U.S. Pat. No. 4,252,211 discloses a loudspeaker with a flat plate diaphragm that is driven by a plurality of spaced apart magnetic drivers. Matsuda et al. U.S. Pat. No. 4,198,550 discloses a flat panel sandwich diaphragm in which the edges are reinforced.
It is, therefore, desirable to develop a compact, planar speaker that consistently and repeatably emits high quality sound over a wide bandwidth throughout the entire life span of the speaker. It is also desirable to develop such a speaker whose sound characteristics are substantially unaffected by changes in temperature, moisture, radiation, and the like. It is also desirable to create such a speaker that is inexpensive to produce and is resistant to the effects of aging. It is also desirable to develop such a speaker that is resistant to sound degradation due to physical damage such as punctures and the like. These and other difficulties of the prior art have been overcome according to the present invention.
It is an object of the present invention to provide a thin, planar speaker that emits a high quality sound output level over a wide bandwidth for the life span of the speaker. It is also an object of the present invention to provide a speaker that can maintain high quality sound reproduction in spite of exposure to environmental changes in temperature, moisture, radiation, and the like, and to the effects of aging that can degrade the performance of the speaker.
It is another object of the present invention to minimize manufacturing costs by providing a planar speaker that is inexpensive to manufacture.
It is yet another object of the present invention to produce a planar speaker that is significantly more resistant to physical damage such as punctures than conventional single wall speaker diaphragms.
A unique flat speaker is disclosed having a flat spaced-apart layered resonator attached to a driver through an outwardly flared radiator. The unique resonator fundamentally comprises a multi-layered structure having an upper layer and a lower layer. The layers are maintained in a spaced-apart relationship by divider walls positioned therebetween. The divider walls and the respective layers define chambers or internal passages within the resonator. The resonator maintains a self-taut state, and the divider walls can be arranged into numerous configurations. In a preferred embodiment the divider walls are arranged in a spaced apart, linear, and parallel relationship, which forms internal passages within the resonator. This configuration also forms open ends of the internal passages at the periphery of the resonator. The resonators can be formed, for example, by extrusion or lay-up procedures. Extrusion procedures where the resonator is fully formed at the moment of extrusion are generally the least expensive of the available resonator formation procedures. Lay-up procedures lend themselves to the formation of resonators with, for example, corrugated, sinuous, or spiral divider walls. In still other embodiments the internal passages defined by the divider walls form individual cells. These cells, defined by the divider walls, can be configured into numerous shapes such as a circle, square, trapezoid, triangle, hexagon, octagon, or the like. In one embodiment the individual cells are shaped in a honeycomb configuration.
The unique resonator of the present invention can be made from many materials such as polymers, metal foils, and cellulose based materials. One or more materials can be used in one resonator, if desired. The flat panel resonator may also be made from homogeneous or heterogeneous composite materials having uniform or non-uniform densities, characteristics, or dimensions along the resonator panel in any direction, or between the layers, or among the divider walls. In a preferred embodiment, the resonator is made by extrusion from a polyimide thermoplastic material. The open ends of the internal passages of the resonator may or may not be sealed at the periphery of the resonator. The sound characteristics of the loudspeaker can be manipulated by, for example, sealing, not sealing, or partially sealing these open ends.
The flat speaker also includes a frame assembly, a mount plate having a plurality of sound relief openings, a driver attached to the mount plate, and a tapered radiator construct having neck and mouth regions with different areas. The resonator is attached at its periphery to the frame assembly and to the mouth portion of the radiator. The mouth portion of the radiator may be attached to either the upper or lower layer of the resonator, as desired. However, a hole must be provided in the resonator when it is attached at its upper layer to the radiator. The neck portion of the radiator is connected to the driver. Although the construction of the resonator is such that the resonator maintains a generally self-taut state, a means for tensionally attaching the resonator to the frame can be used, if desired. Such tensionally attachment means can, for example, take the form of tensor rods. The radiator vibrates responsive to the vibration of the driver. The radiator, in turn, causes the resonator to vibrate. The radiator is preferably a three dimensional tapered object in the form of a right circular shell with the surface of the shell being defined as a surface of revolution about an axis of revolution. The radiator can be configured into various shapes, such as frusto-conical, parabolic, bell, or the like. Preferably, the radiator is attached slightly off from the geometric center of the resonator in order to eliminate the cancellation of sound waves propagating across the resonator.
The flat speaker described herein produces significant improvements in sound quality and volume output, and durability compared to conventional flat speakers.